J. Belloni, M. Clerc, P. Goujon, and E. Saito
2848
not distinguish between two possibilities. Either a preexisting trap containing H20 does not absorb in the above range, or the rotational relaxation time of H20 is faster than that of CzH50H. One has to probe shorter time scales to establish this point. (2) I would like to raise the question of the nature of the preexisting traps in polar glasses and in polar liquids. Obviously, in a glass we have a distribution of local potentials while in the liquid rotational fluctuations will result in essentially the same effect. In your language this corresponds to an electron moving in a “rough” potential. To be more precise, such a situation corresponds to the Anderson problem of an electron moving in a fluctuating potential. Only when the energetic spread of the potentials exceeds a certain “critical” value, relative to the band width, will localization occur. Thus the potential not only has to be “rough” but must also exhibit a large spatial energetic spread.
L. KEVAN.(1)We did try to distinguish these two possibilities by looking at shorter times. With a 40-nsec pulse there is little change in the spectra in Figure 1 for the first 200 nsec after the start of the pulse and then all of the spectra with various mole fractions of water begin to shift on about the same time scale to nearly the same final spectrum. On this basis we think that the “initial” spectra presented are initial and characteristic of different types of solvation shells for the electron. ,(2) Yes, the “rough” potential I speak of corresponds to a spread of potentials both energetically and spatially. However, we still have no real understanding of the initial localization process of an electron in this “rough” potential. Perhaps it is mainly due to strong scattering events and perhaps resonance capture interactions are important for certain energy ranges.
S. A. RICE. Have electron scavengers been added to glassy ethanol to discover whether the infrared absorption is characteristic of electron-geminate cation pairs? L. KEVAN. No scavenger experiments have been done in the ethanol-water glasses in these experiments. In earlier work (L. Kevan, J . Chem. Phys., 56, 838 (1972)) we did add biphenyl as a
scavenger. There the intensity of the electron band was decreased because some of the electrons were scavenged, but the shape and time profile of the infrared absorption did not change.
M. NEWTON.How did you simulate the original solvent configuration for the partially solvated electron? Is it not surprising that further solvation leads to an increase in the effective radius for the dipolar interaction? L. KEVAN.This question refers to our study of average distance changes in the process or solvation (H. Hase et al., J . Chem Phys., 62, 985 (1975)). We did not have to assume any particular solvent configuration for the partially solvated electron. We simply simulated the matrix ENDOR line for these electrons. Further solvation leads to an increase in the average distance between the electron and the CH matrix protons, Le., those protons that do not overlap significantly with the electron wave function. The CH dipole has its H end negative so as solvation proceeds the H end of the CH dipole moves slightly away from the electon. The OH grotons in alcohol matrices have opposite polarity and probably move the other way. We do not obtain direct information about the OH protons from matrix ENDOR, because there is enough isotropic hyperfine coupling to these protons to move their ENDOR signal away from the free proton frequency where the matrix ENDOR signal appears. N. R. KESTNER.Is your experiment simply sampling initially only those sites which already contain one water molecule or do you think you see those species in which the electron drags one water molecule into the first coordination layer? How can one be sure which is correct?
L. KEVAN.We have assumed that the experiment samples those electrons which already have a water molecule in the first solvation shell from statistical considerations. In a glassy matrix of high macroscopic viscosity, such as we have, we do not think that the electron will exert much “drag” on water molecules outside the first or perhaps the second solvation shell, especially when the other molecules (ethanol) are also strongly polar.
Solvation Time of Electrons in Liquid Ammonia J. Belloni,t M. Clerc,* P. Goujon; and E. Saito DRA. SRIRMA. CEN Saclay, 91 190 Gif-sur-Yvette,France (Received August 1, 1975) Publication costs assisted by Service de Documentation. CEN Saclay
The phenomenon of solvation of an electron depends either on the presence of preexisting positive potential wells and/or on the orientation of the surrounding molecules b y the field arising from the electron’s charge. In our studies we have tried t o evaluate the solvation t i m e for an excess electron in liquid a m m o n i a at low t e m p e r a t u r e (-50OC). The experiment consists of exciting a K-NHa solution b y a picosecond laser pulse i n the absorption b a n d of t h e solvated electron present, a n d determining the t i m e of the whole process of detrapping and recovering of the initial absorbance. + Laboratoire de Physico-chimie des Rayonnements, Associkan CNRS, 91605 Ordy. 1 DGI-SEPCP, CEN Saclay. The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
W e used a d y e laser (Rhodamine 6G) p u m p e d by an abrasive lamp, producing a 1.5-psec train consisting of 300 to 400 pulses, each of 5 to 10 psec a n d separated b y 4-nsec intervals. The wavelength of t h e b e a m is 610 n m a n d the intensity is -2 x 1 O I 4 photons per pulse. The analyzing light (xenon lamp) and the laser beam fall on the 1 mm cell with almost the s a m e incident angle b u t adjusted so that the laser beam does n o t fall o n the spectrograph slit (Figure 1).The K-NH:i solutions were i n the con-
Sohratiin Time of Electrons in LiquM Ammonia
2849
3
centration range to lo-* M. T h e resolution of the spectrograph allowed us to observe the wavelength range 610-630 nm. T h e variation of absorbance with time was followed by ultrafast spectroscopy using a picosecond streak camera. T h e highest scanning rate was 125 pseclcm corresponding to a resolved time of 5 psec. Although we have not been able to avoid successive reflections of the laser beam on the thick walls of the cell, the main observation derived from these experiments is that the transient transparency due to bleaching and recovering of the initial absorbance lasts no longer than the pulse itself. Supposing that the bleaching process is much shorter than that of solvation, we conclude that the solvation time of electrons in liauid NH3 at -50°C is 6 5 psec (Fiaure 2).
Discussion U. SCHINDEWOLF. I would like to raise the question about the
I
I
-830
I620
100 ps
Flgure 2. Bleaching of the solvated electron in lQuM N& (-SOOC).
p r e s s you are observing: by excitation with about 600-nm light you might go to the 2p, 3p, 4p,. . . state with dipole orientation or structure of the ground state. So I would suppose you observe the relaxation back to the ground sate. rather than that of solvation. because with the light energy you apply an electron probably can: not be kicked out of ita solvation shell. J. JORTNER. Optical excitation of the solvated electron band can result in two major types of subsequent relaxation processes, depending on the excitation energy. (a) Excitation of a boundhound transition, e.g., 1s 2p will result in multiphonon nonradiative relaxation within the single trapping center. (b) Excitation above the threshold for ionization will result in a quasi-free electron which will then be localized. The present experiments come. spond to case b, as the excitation energy is 2.0 eV (i.e., 6100 A).
-
J. BELLONI. I agree that the energy of the exciting photons is high enough to excite the transition to the continuum, It is likely that in this region of the spectrum'the time to recover absorption correspondsto a complete solvation time instead ofobservationsat about 1000 nm where solvation could he only a partial one (work from Bell Laboratories).
The Jownaf of PhyJlcalChemirhy, Vol. 79. No. 26, 1975
